1. Field of the Invention
The present invention relates to a method and system for manipulating information on videotapes and, more particularly, to a method and system for reading and controllably recording data on videotape, such as vertical interval time code information, that is especially useful when editing videotapes.
2. State of the Art
In modern videotape recording technology, it is commonplace to extensively edit videotapes. In practice, editing processes are usually described as being either "insert" or "assemble" operations. In an insert operation, new information on videotape is spliced into the midst of existing information. In an assemble operation, new information on videotape is added at the end of other information. In either editing operation, it is desirable that editing orientation information on videotape is consistent before and after edit transitions.
Storage of editing orientation information on videotapes can be provided in so-called "vertical blanking intervals." Normally, vertical blanking intervals correspond to the time required by cathode ray emitters to "return scan" from the bottom to the top of a screen; during return scan periods, the cathode ray scanners do not produce luminescence on the screen. Because television signals are transmitted continuously but the operation of cathode ray emitters is discontinuous, the return scan periods are appropriately referred to as vertical blanking intervals. In modern practice, vertical blanking intervals may include twenty or more lines that contain signal information such as video captions for hearing-impaired individuals and, most importantly for present purposes, vertical interval time code (VITC) information.
VITC information is normally recorded in vertical blanking intervals to identify the time at which a particular frame occurs after a frame identified as "frame zero". (Frame zero often, but not necessarily, coincides with the beginning of a recording.) For example, the 2055th frame following frame zero might be designated as 00:01:08:15, indicating that the frame follows frame zero by 1 minute and 8.5 seconds when the tape is played at a normal operating speed of thirty frames per second; that is, the first two digits in this code designate hours in decimal notation, the next two digits designate minutes, the next two digits designate seconds, and the final two digits designate numbers of frames, Providing VITC information is, therefore, tantamount to identifying video frames by addresses sequential from frame to frame.
In practice, VITC information is similar to well known longitudinal time code information except it contains one "sign" bit that identifies which interlaced field of video information the time code information is extracted from.
Precise code formats for VITC information have been specified by standards, such as ANSI/SMPTE 12M-1986 sponsored by the American National Standards Institute (New York, NY) and by the Society of Motion Picture and Television Engineers. In this standard format, VITC information comprises 90 digital bits consisting of 9 bytes with two synchronizing bits between each byte encoded into a line of video information; included within the 90 bits are two initial synchronizing bits and eight cyclical redundancy check code (CRC) bits at the end of the line. Other standard code formats for VITC information exist, however, in addition to the American National Standard; for example, the European Broadcaster's Union standard sets forth a somewhat different format for VITC code information. Typically under the various standards, VITC information is recorded redundantly on only two non-adjacent lines in a vertical blanking interval, and is identified as VITC information by unique synchronizing bit sets embedded in the information.
Although standards specify code formats for VITC information, the standards provide substantial flexibility as to placement of VITC information in vertical blanking intervals. Consequently, in videotape editing, selection of placement of VITC information is a matter of choice with individual editors, and actual line placement of VITC information may differ from machine to machine or from editor to editor. For example, one editor may choose to record VITC information on the 13th and 15th lines in vertical blanking intervals, and another may elect to record VITC information on the 17th and 19th lines.
Although flexibility as to placement of VITC information in vertical blanking intervals is highly desirable for some editing purposes, the flexibility can cause problems when videotapes are re-edited or replayed. In part, such problems arise because conventional devices for reading VITC information from videotape, usually called "VITC readers", do no provide identification of addresses of lines from which VITC information is read. In the absence of such address information, it is conventional editing practice to arbitrarily choose line addresses to record VITC information without regard for line addresses at which VITC information may have been previously recorded on the videotape being edited. Thus, in conventional practice, the location of VITC information in terms of line addresses is highly unpredictable. One complication arising from such practice is that, during editing, VITC information may be recorded over important non-VITC information in vertical blanking intervals. For instance, an editor may inadvertently write VITC information over closed captioned information, causing loss or distortion of captions during replay of edited videotape.
In view of the preceding, it can be appreciated that instances may exist where previously recorded VITC information co-exists with newly recorded VITC information on an edited videotape; for example, previously recorded VITC information may appear on the 13th and 15th lines in vertical blanking intervals of an edited videotape and newly recorded VITC information may be recorded on the 17th and 19th lines of the same tape. This can cause difficulties during editing because conventional VITC readers locate only the first valid line of VITC information in vertical blanking intervals and ignore all subsequent lines. Thus, when re-editing a videotape, inconsistencies may occur if a VITC reader should sometimes miss the newly recorded VITC information and, instead, read the originally recorded information; it is also possible that a VITC reader might at times read the newly recorded VITC information and, at other times, read the previously recorded VITC information.
Further with regard to the state of the art, it should be understood that conventional devices exist for assuring that newly recorded VITC information is numerically-sequential with previously recorded VITC information across edit transitions. Such devices simplify editing tasks, because the sequential VITC information conveniently guides an editor to a desired point on an edited videotape.